Method and apparatus for surface profiling of materials and calibration of ablation lasers

ABSTRACT

Method and apparatus are provided for measuring the surface profile of a sample. The method and apparatus provide light from a light source through a beam splitter to form two split beams, direct the split beams onto a sample surface and a reference surface respectively, reflect the split beams back through the beam splitter, and direct the split beams towards an imaging system. A surface profiling apparatus for measuring the surface profile of the sample is also provided. The apparatus includes a light source for generating a source beam, beam splitting means positioned in the path of the source beam for splitting the source beam into split beams, a reference surface, a sample surface allowing the split beams to traverse separate paths and return to the beam splitting means, reference surface positioning means for positioning the reference surface, and viewing means for imaging combined beams. An apparatus for calibrating a laser for the ablation of a material including the surface profiling apparatus is also provided.

This is a continuation of copending International application No.PCT/AU98/00568 filed Jul. 17, 1998.

The present invention relates to the surface profiling of materials, forexample as in the laser processing or ablation of materials, or asneeded in the calibration and performance checking of the laserapparatus used in operations on the corneal tissue of the eye for thecorrection of refractive errors.

The invention will be described by reference to two operations for thecorrection of refractive errors, photorefractive keratectomy (PRK) andlaser in-situ keratomeleusis (LASIK), but the invention may be used tomeasure the surface profile of a wide range of materials or to calibratelasers for a variety of medical and industrial applications.

To ensure that the correct profile is etched onto a patient's corneaduring PRK or LASIK, the surgical laser must first be calibrated. Thisprocess imparts an accurate picture of how the laser will ablate thecornea. The corneal surface may be ablated to effect a myopic, hyperopicor astigmatic correction. Myopic corrections should produce a new,flatter curvature, while hyperopic corrections should remove morematerial around the edge of the area to be ablated.

One of the current methods used to perform the calibration procedureinvolves etching the surface of a plastic polymer such aspolymethyl-methacrylate (PMMA). The etched surface is examined by aninstrument known as a lensometer. This instrument determines the powerof the resultant ‘lens’ in diopters. The reading can then be compared tothe desired refractive correction. Discrepancies between the desired andachieved readings indicate that the laser needs to be adjusted, by afactor proportional to the difference between the lensometer reading andthe desired surgical correction (see U.S. Pat. No. 5,261,822).

Another method of calibration is described in U.S. Pat. No. 5,261,822.This patent illustrates the use of a calibration block that can beexamined by visual inspection. It teaches the use of a plurality of thincoatings of PMMA of progressively increasing thickness, layered over asolid substrate of the same material. Each layer may be doped with adifferently coloured or fluorescent material. When the cavity ofmaterial ablated by the laser is viewed from above, a pattern of circlesis visible. A correctly calibrated laser should produce patterns ofconcentric circles, whereas patterns of eccentric circles indicate thatthe laser is not correctly calibrated. However, the result is usuallyjudged subjectively and this technique provides only a crude predictionof the shape created during a refractive correction.

The above laser calibration methods suffer from a number ofdisadvantages. PMMA does not necessarily mimic the ablationcharacteristics of corneal tissue, and different brands of PMMA ablateat different rates (P.P. van Saarloos and I. J. Constable, J. Appl.Phys. 68(1) (1990) 377). Further, different brands of lasers ablate atdifferent fluences, where the ratio of ablation rates of tissue andplastic are different. Nor does the lensometer provide an accuratereading of the ablation surface. The shape desired to be etched on thecornea does not necessarily produce an accurate lens shape when ablatedinto plastic. The ablated surface is usually aspheric, and may beinaccurately read. This means that a lensometer reading does not give anabsolute measure of laser performance, and in some cases the measurementis meaningless. This method can therefore only give an approximatereading of surface curvature. Lensometer readings are also timeconsuming.

Other known methods to measure ablated surface profiles include the useof interferometry, or include scanning the ablated surface with ascanning electron microscope, a confocal microscope or surface contactneedles. Devices according to these known methods are, however, costlyand of prohibitive size, and impractical to cover the range of shapesproduced by refractive lasers. There exists, therefore, a demand for anaccurate, low cost device for performance analysis and calibration ofrefractive lasers to ensure appropriate shapes are etched onto thesurface to be ablated.

It is an object of the present invention to provide a new and improvedmethod and apparatus for surface profiling of materials and calibrationof ablation lasers that can more accurately and reliably examine thesurface of an ablation.

According to the present invention, therefore, there is provided amethod for measuring the surface profile of a sample, said methodincluding:

directing light from a light source through a beam splitter to form twosplit beams;

directing said split beams onto a sample surface and a reference surfacerespectively;

reflecting the split beams back through the beam splitter; and

directing said split beams towards an imaging system.

Preferably the method is for use in calibrating a laser ablationapparatus for ablation of a material by measuring the result of anablation of the sample.

The method may include reflecting said light from a mirror and/orfocussing said light to minimise space requirements.

Preferably the light source includes a light emitting diode.

Alternatively the light source is a source of white or near infra-redlight.

Preferably the sample surface is a plastic polymer that ablates at asubstantially constant fraction of the ablation rate of said materialover the range of fluences used in ablating said material, andpreferably the fraction equals 1.0.

The material may be biological material.

The biological tissue may be corneal tissue, and the method includeablating said material in a surgical procedure, in which case thefluences are preferably in the range 50-800 mJ/cm², and more preferablyin the range 120-250 mJ/cm².

Preferably the reference surface is a flat mirror or a flat surface.

The reference surface may be mounted on a pendulum including a pluralityof substantially parallel sheets of flexible material.

The method may include moving the reference surface by means of aspeaker or voice coil.

Preferably the imaging system includes a CCD video camera.

The method may include measuring said surface profile, comparing saidmeasurement with a predicted profile, and determining an indicator ofthe safety or predictability of ablation performed on said sample foruse in a surgical procedure.

Preferably the reference surface positioning means includes a voice coildriver and a position sensor.

The method may include transferring the calibration profile informationascertained by said method into a laser system control computing device,to allow the self correction of the calibration and shape controls ofthe laser system.

The method may also include communicating with a topography measuringdevice for measuring the topography of the front surface of a human oranimal eye in order to combine the results of a calibration measurementin plastic and the results of a topography measurement, and predictingfrom said calibration and topography results the post laser treatmentshape of the eye.

The present invention also provides a surface profiling apparatus formeasuring the surface profile of a sample, the apparatus including:

a light source for generating a source beam;

beam splitting means positioned in the path of the source beam forsplitting said source beam into split beams;

a reference surface;

a sample surface allowing said split beams to traverse separate pathsand return to said beam splitting means;

reference surface positioning means for positioning the referencesurface; and

viewing means for imaging combined beams.

The apparatus may include focussing optical elements to concentrate theintensity of said light, and a mirror, said optical elements and saidmirror located between said light source and said beam splitting means.

Preferably the light is white light or near infra-red light.

The light source may include a halogen bulb, or a light emitting diode(LED).

Preferably said LED has a maximum intensity in the red to infra-redportion of the spectrum.

The reference surface may be a flat mirror or a flat surface.

The imaging system preferably includes a CCD video camera.

Preferably the reference surface positioning means includes a voice coildriver and a position sensor.

Preferably the position sensor includes a known sample.

Preferably the position sensor includes a mirror or optical element thatallows both the known sample and the plastic sample being measured to beviewed by means of the imaging system simultaneously or alternately.

In one form of the invention, the position sensor is a capacitance orinductance position sensor.

Preferably the voice coil driver is similar to that used in aloud-speaker.

The position sensor may be an opto-electric sensor including aphotodiode with an amplification system and an additional LED, whereinthe sensor uses the intensity of the additional LED, and said additionalLED is positioned to reflect light at an angle from the referencesurface, or any surface moving with the reference surface, to thephotodiode.

Preferably the position sensor is one of a plurality of positionsensors.

Preferably the plurality of position sensors includes a plurality oftypes of position sensor.

In one embodiment, the reference surface positioning means includes aloud-speaker.

Preferably the loud-speaker is used as or constitutes a displacementdriver for the reference surface.

Preferably the reference surface is mounted on a pendulum including aplurality of substantially parallel sheets of flexible material.

The invention also provides an apparatus for calibrating a laser for theablation of a material including the surface profiling apparatusdescribed above.

The sample surface may comprise a plastic polymer that ablates at asubstantially constant fraction of the ablation rate of said materialover the range of fluences used in ablating said material, andpreferably the fraction equals 1.0.

The material may be biological material, including for example cornealtissue, and the apparatus may be for ablating the material in a surgicalprocedure (such as PRK or LASIK). In these cases the fluences arepreferably in the range 50-800 mJ/cm² and more preferably in the range120-250 mJ/cm².

In one particular embodiment, the apparatus includes a laser means,wherein the apparatus is for calibrating and/or checking the lasermeans, and includes communication means for communicating with, acomputer controlled laser means, whereby the laser means can usecalibration profile information obtained by the calibration apparatus toself correct the calibration and shape controls of said laser means. Inthis embodiment, the laser means may be for use in PRK or LASIKoperations of the cornea of the eye to correct refractive errors.

The apparatus may include a corneal topography measuring means formeasuring the topography of the front surface of a human or animal eyeand communication means for communicating with said topography measuringmeans, for predicting post laser treatment eye topography fromcalibration measurements in plastic and topography measurements of theeye, and may further include display means for displaying the post lasertreatment corneal topography predicted by means of the apparatus.

In order that the invention may be more fully explained, some preferredembodiments will be described, by way of example, with reference to theaccompanying drawings in which:

FIG. 1A is a diagrammatic plan view of an ablation pattern formed by alaser source directed onto the surface of a plastic sample surface;

FIG. 1B is a cross section through A—A of FIG. 1A; and

FIG. 2 is a schematic view of a calibration apparatus according to apreferred embodiment of the present invention.

FIGS. 1A and 1B show a typical myopic or myopic/astigmatic ablationpattern etched onto the surface of a plastic sample surface. Theablation pattern may have been etched by an excimer, solid state orother type of laser suitable for refractive correction.

Referring to FIG. 2, the first arrangement of the apparatus includes ared light source in the form of light emitting diode 2. Alternativelythe light source may be a general purpose halogen bulb. The light 4passes through a beam splitter 6 where two separate beams are formed.Some of the light is directed onto the ablated sample 8, which is aplastic polymer that ablates at the same rate as corneal tissue over therange of laser fluences used in corneal ablation procedures, 120 to 250mJ/cm². The rest of the light is directed onto a reference surface 10comprising a mirror or other flat surface which is scanned back andforth. Both the ablated plastic sample 8 and the reference surface 10reflect or scatter the light back to the beam splitter 6. Some of thereflections from the sample 8 and the reference surface 10 bounce offthe beam splitter 6 and disappear. The remaining combined beam isdirected through the beam splitter 6 towards a CCD video camera 12, forexample a COHO 11C0 video camera or the like. The reference surface 10is scanned to adjust the beam path length of the light going back to thecamera 12. When the light beam path length from the sample surface 8matches the path length to the reference surface 10, interferencepatterns will be formed.

For an ablated sample such as that of FIG. 1, when viewed through thecamera 12, circular interference patterns are imaged for good,non-astigmatic, myopic ablations. A smaller circular pattern is producedat the deepest point of the ablated surface, when the reference surface10 is further away from the beam splitter 6. Progressively largercircular patterns are produced as shallower ablations are encountered.

However, interference patterns can only be produced when the referencesurface 10 and a point on the ablated sample 8 are at the same opticalpath distance from the video camera 12. The reference surface 10 musttherefore be movable to allow the imaging of different ablation depths.A voice coil driver 14 moves the reference surface 10 back and forth,while an opto-electronic sensor 16 (or, in other embodiments, acapacitance or inductance position sensor), such as a photodiode with anamplification system, senses the spatial positioning of the referencesurface 10. Voice coil driver 14 and position sensor 16 therefore allowpositioning, with feedback from the reference surface 10 in relation tothe ablated sample 8.

An alternative embodiment involves the use of optical rather thanmechanical position measurement. In this embodiment, a known sample inthe form of a wedge shaped object 18, and a small mirror 20, are used todetect the positioning of the reference surface 10. Voice coil 14 isagain used to drive the reference surface 10. In this embodiment, theknown sample 18 features a sloping surface 22 that reflects the minimumand maximum movement of the reference surface 10. However, positionsensor 16 may additionally be used in this embodiment.

The calibration device as described above is preferably connected to acomputer 24. This computer 24 can calculate the shape of the ablatedsample surface 8, display the shape in a three dimensional form, comparethe actual shape to a desired shape and issue a “go/no go” message,indicating that a good calibration or a laser problem has been detected,respectively. The computer may also be joined to a laser system orcorneal topography device 26. The calibration device can thereforeexchange information concerning the ablated profile with the lasersystem. The information provided about the measured profile produced canthen be interpreted, and used to alter the parameters of the lasersystem so that the desired corneal profile is produced in its nextablation.

Apparatus for performing topographic profiling of the cornea may also beincluded in a preferred embodiment. This apparatus may be used tomeasure the original profile of a corneal surface and then import themeasured ablation profile from the calibration apparatus of the presentinvention. The corneal topography that may be expected if a laserablation procedure were performed on a cornea, based on the calibrationdata, may then be calculated and displayed. Alternatively, thecalibration apparatus may read the corneal topographic data, andcalculate and display on computer 24 the resultant corneal shape thatwould be created if the laser was used on the eye.

Thus, the present invention may be used to calibrate lasers used, forexample, in the improvement of eyesight or other medical, dental orcosmetic procedures where the accurate ablation of tissue is required.

Modifications within the spirit and scope of the invention may bereadily effected by a person skilled in the art. Such modifications mayinclude swapping positions of the sample and reference surfaces. It isto be understood, therefore, that this invention is not limited to theparticular embodiments described by way of example hereinabove.

What is claimed is:
 1. A method for calibrating laser ablationapparatus, including: ablating a sample; measuring the surface profileof said sample by: directing light from a light source through abeam-splitter to form two split beams; directing said split beamsrespectively onto the ablated surface of said sample and also onto areference surface; reflecting the split beams from said ablated surfaceand said reference surface, respectively, and forming an interferencesignal from said reflected split beams; and detecting said interferencesignal and therefrom determining the surface profile of said samplesurface; and calibrating said laser ablation apparatus on the basis ofsaid determined surface profile.
 2. A method as claimed in claim 1,wherein said sample surface is a plastic polymer.
 3. A method as claimedin claim 1, including reflecting said light from a mirror and/orfocussing said light to minimise space requirements.
 4. A method asclaimed in claim 1, wherein said light source includes a light emittingdiode.
 5. A method as claimed in claim 1, wherein said light source is asource of white or near infra-red light.
 6. A method according to inclaim 1 for calibrating said apparatus for use in ablating apredetermined material, wherein said sample has an ablation rate that isa substantially constant fraction of the ablation rate of said materialto be ablated over the range of fluences used in ablating said material.7. A method as claimed in claim 6, wherein said fraction equals 1.0. 8.A method as claimed in claim 6, wherein said material is biologicaltissue.
 9. A method as claimed in claim 8, wherein said biologicaltissue is corneal tissue.
 10. A method as claimed in claim 8, incombination with the further step of ablating said material in asurgical procedure.
 11. A method as claimed in claim 8, wherein saidbiological tissue is corneal tissue and said fluences are in the range50-800 mJ/cm².
 12. A method as claimed in claim 11, wherein saidfluences are in the range 120-250 mJ/cm².
 13. A method as claimed inclaim 1, wherein said reference surface is a flat mirror or a flatsurface.
 14. A method as claimed in claim 1, wherein said referencesurface is mounted on a pendulum including a plurality of substantiallyparallel sheets of flexible material.
 15. A method as claimed in claim1, including moving said reference surface by means of a speaker orvoice coil.
 16. A method as claimed in claim 1, wherein saidinterference signal is detected with an imaging system that includes aCCD video camera.
 17. A method as claimed in claim 1, includingmeasuring said surface profile, comparing said measurement with apredicted profile, and determining an indicator of the safety orpredicability of ablation performed on said sample for use in a surgicalprocedure.
 18. A method as claimed in claim 1, including adjusting thecalibration and shape controls of the laser ablation apparatus.
 19. Amethod as claimed in claim 1, including communicating with a topographymeasuring device for measuring the topography of the front surface of ahuman or animal eye in order to combine the results of a calibrationmeasurement in plastic and the results of a topography measurement, andpredicting from said calibration and topography results the post lasertreatment shape of the eye.
 20. A method according to claim 1 whereinsaid reflected split beams are directed back through said beamsplittermeans to form said interference signal.
 21. An apparatus for calibratinglaser ablation apparatus, comprising: a light source for generating asource beam; beam-splitter means positioned in the path of the sourcebeam for splitting said source beam into split beams; a referencesurface positioned to reflect one of said split beams back to saidbeamsplitter means for forming an interference signal with another ofsaid split beams reflected back to said beamsplitter means by a surfaceof a sample ablated by said laser ablation apparatus; reference surfacepositioning means including a voice coil driver for positioning thereference surface; means to detect the position of said referencesurface, to which detection means the voice coil driver is responsive;means for imaging said interference signal; and means for determining,from said imaged interference signal, the surface profile of said samplesurface and for calibrating said laser ablation apparatus on the basisof said determined surface profile.
 22. An apparatus as claimed in claim21 including focusing optical elements to concentrate the intensity ofsaid source beam, and a mirror, said optical elements and said mirrorlocated between said light source and said beamsplitter means.
 23. Anapparatus as claimed in claim 21, wherein said light source provideswhite light or near infra-red light.
 24. An apparatus as claimed inclaim 21, wherein said light source includes a halogen bulb, or a lightemitting diode (LED).
 25. An apparatus as claimed in claim 24, whereinsaid LED has a maximum intensity in the red to infra-red portion of thespectrum.
 26. An apparatus as claimed in claim 21, wherein saidreference surface is a flat mirror or a flat surface.
 27. An apparatusas claimed in claim 21, wherein said imaging means includes a CCD videocamera.
 28. An apparatus as claimed in claim 21, wherein said positiondetection means includes a known sample.
 29. An apparatus as claimed inclaim 28, wherein said position detection means includes a mirror oroptical element that allows both the known sample and said sample beingmeasured to be viewed by means of the imaging system simultaneously oralternately.
 30. An apparatus as claimed in claim 21, wherein saidposition sensor is a capacitance or inductance position sensor.
 31. Anapparatus as claimed in claim 21, wherein said voice coil driver issimilar to that used in a loud-speaker.
 32. An apparatus as claimed inclaim 21, wherein said position detection means is an opto-electricsensor including a photodiode with an amplification system and anadditional LED, wherein the sensor uses the intensity of the additionalLED, and said additional LED is positioned to reflect light at an anglefrom the reference surface, or any surface moving with the referencesurface, to the photodiode.
 33. An apparatus as claimed in claim 21,wherein said position detection means comprises a plurality of positionsensors.
 34. An apparatus as claimed in claim 33, wherein said pluralityof position sensors includes a plurality of types of position sensor.35. An apparatus as claimed in claim 21, wherein said reference surfacepositioning means includes a loud-speaker.
 36. An apparatus as claimedin claim 35, wherein said loud-speaker is used as or constitutes adisplacement driver for the reference surface.
 37. An apparatus asclaimed in claim 21, wherein the reference surface is mounted on apendulum including a plurality of substantially parallel sheets offlexible material.
 38. An apparatus as claimed in claim 21, furtherincluding said sample, wherein said sample surface comprises a plasticpolymer.
 39. An apparatus as claimed in claim 38 for calibrating saidlaser ablation apparatus for use in ablating a predetermined material,wherein said sample has an ablation rate that is a substantiallyconstant fraction of the ablation rate of said material to be ablatedover the range of fluences used in ablating said material.
 40. Anapparatus as claimed in claim 39 wherein said material is biologicaltissue.
 41. An apparatus as claimed in claim 40, wherein said biologicaltissue is corneal tissue.
 42. an apparatus is claimed in claim 40, incombination with apparatus for laser ablating said biological tissue ina surgical procedure.
 43. An apparatus as claimed in claim 42, whereinsaid surgical procedure is PRK or LASIK.
 44. An apparatus as claimed in40, wherein said fluences are in the range 50-800 mJ/cm².
 45. Anapparatus as claimed in claim 44, wherein said fluences are in the range120-250 mJ/cm².
 46. An apparatus as claimed in claim 21, including laserablation apparatus, said means for determining said surface profile andfor calibrating said laser ablation apparatus being in communicationwith said laser ablation apparatus for adjusting the calibration andshape controls thereof.
 47. An apparatus as claimed in claim 21,including a corneal topography measuring means for measuring thetopography of the front surface of a human or animal eye andcommunication means for communicating with said topography measuringmeans, for predicting post laser treatment eye topography fromcalibration measurements in plastic and topography measurements of theeye.
 48. An apparatus as claimed in claim 21, including a cornealtopography measuring means for measuring the topography of the frontsurface of a human or animal eye and communication means forcommunicating with said topography measuring means, for predicting postlaser treatment eye topography from calibration measurements in plasticand topography measurements of the eye.
 49. An apparatus as claimed inclaim 48, including display means for displaying the post lasertreatment corneal topography predicted by means of the apparatus. 50.Apparatus for calibrating laser ablation apparatus, comprising: a lightsource for generating a source beam: beamsplitter means positioned inthe path of the source beam for splitting said source beam into splitbeams; a reference surface positioned to reflect one of said split beamsfor forming an interference signal with another of said split beamsreflected by a surface of a sample ablated by said laser ablationapparatus; reference surface positioning means for positioning thereference surface; means for imaging said interference signal; and meansfor determining, from said imaged interference signal, the surfaceprofile of said sample surface and for calibrating said laser ablationapparatus on the basis of said determined surface profile.
 51. Anapparatus as claimed in claim 50, including focussing optical elementsto concentrate the intensity of said light, and a mirror, said opticalelements and said mirror located between said light source and saidbeam-splitter means.
 52. An apparatus as claimed in claim 50, whereinsaid light source provides white light or near infra-red light.
 53. Anapparatus as claimed in claim 50, wherein said light source includes ahalogen bulb, or a light emitting diode (LED).
 54. An apparatus asclaimed in claim 53, wherein said LED has a maximum intensity in the redto infra-red portion of the spectrum.
 55. An apparatus as claimed in anyone of claim 50, wherein said reference surface is a flat mirror or aflat surface.
 56. An apparatus as claimed in claim 50, wherein saidimaging means includes a CCD video camera.
 57. An apparatus as claimedin claim 50, wherein said reference surface positioning means furtherincludes a position sensor.
 58. An apparatus as claimed in claim 57,wherein said position sensor includes a known sample.
 59. An apparatusas claimed in claim 58, wherein said position sensor includes a mirroror optical element that allows both the known sample and said samplebeing measured to be viewed by means of the imaging systemsimultaneously or alternately.
 60. An apparatus as claimed in claim 57,wherein said position sensor is a capacitance or inductance positionsensor.
 61. An apparatus as claimed in claim 57, wherein said positionsensor is an opto-electric sensor including a photodiode with anamplification system and an additional LED, wherein the sensor uses theintensity of the additional LED, and said additional LED is positionedto reflect light at an angle from the reference surface, or any surfacemoving with the reference surface, to the photodiode.
 62. An apparatusas claimed in claim 57, wherein said position sensor is one of aplurality of position sensors.
 63. An apparatus as claimed in claim 62,wherein said plurality of position sensors includes a plurality of typesof position sensor.
 64. An apparatus as claimed claim 50, wherein saidvoice coil driver is similar to that used in a loud-speaker.
 65. Anapparatus as claimed in claim 50, wherein said reference surfacepositioning means includes a loud-speaker.
 66. An apparatus as claimedin claim 65, wherein said loud-speaker is used as or constitutes adisplacement driver for the reference surface.
 67. An apparatus asclaimed in claim 50, wherein the reference surface is mounted on apendulum including a plurality of substantially parallel sheets offlexible material.
 68. An apparatus as claimed in claim 67 forcalibrating said laser ablation apparatus for use in ablating apredetermined material, wherein said sample has an ablation rate that isa substantially constant fraction of the ablation rate of said materialto be ablated over the range of fluences used in ablating said material.69. An apparatus as claim 68, wherein said fraction equals 1.0.
 70. Anapparatus as claimed in claim 68 or 69, wherein said material isbiological tissue.
 71. An apparatus as claimed in claim 70, wherein saidbiological tissue is corneal tissue.
 72. An apparatus is claimed ineither claim 70 or 71, in combination with apparatus for laser ablatingsaid biological tissue in a surgical procedure.
 73. An apparatus asclaimed in claim 72, wherein said surgical procedure is PRK or LASIK.74. An apparatus as claimed in claim 70, wherein said fluences are inthe range 50-800 mJ/cm².
 75. An apparatus as claimed in claim 74,wherein said fluences are in the range 120-250 mJ/cm².
 76. An apparatusas claimed in claim 50, further including said sample, wherein saidsample surface comprises a plastic polymer.
 77. An apparatus as claimedin claim 50, wherein the laser source means is for use in PRK or LASIKoperations of the cornea of the eye to correct refractive errors.
 78. Anapparatus as claimed in claim 50, including a corneal topographymeasuring means for measuring the topography of the front surface of ahuman or animal eye and communication means for communicating with saidtopography measuring means, for predicting post laser treatment eyetopography from calibration measurements in plastic and topographymeasurements of the eye.
 79. An apparatus as claimed in claim 78,including display means for displaying the post laser treatment cornealtopography predicted by means of the apparatus.
 80. An apparatus asclaimed in claim 50, including laser ablation apparatus, said means fordetermining said surface profile and for calibrating said laser ablationapparatus being in communication with said laser ablation apparatus foradjusting the calibration and shape controls thereof.
 81. An apparatusas claimed in claim 80, wherein the laser ablation apparatus is for usein PRK or LASIK operations of the cornea of the eye to correctrefractive errors.